(fs) Increase buffer size used with getmntent

Currently jni func getmntent uses library's getmntent_r with work buffer of size 1024, allocated on stack.

JNIEXPORT jint JNICALL
Java_sun_nio_fs_LinuxNativeDispatcher_getmntent(JNIEnv* env, jclass this,
    jlong value, jobject entry)
{
    struct mntent ent;
    char buf[1024];
....
 m = getmntent_r(fp, &ent, (char*)&buf, buflen);

In some circumstances it's not enough. for example in docker, with overlayfs we can easily go greater than 1024 bytes per line

need to increase that size, it's better to go with library's getmntent, which will allocate 4096 on glibc and as much as needed on musl. In this case java will be on par with other system utils.

example of entry in /proc/mounts which causes parsing failure in java

overlay / overlay rw,relatime,lowerdir=/var/lib/docker/overlay2/l/4OJWMLVEPUYBIINATIJYD5AR4A:/var/lib/docker/overlay2/l/BDPV7JUZMLFT2UTSHLSXAAEXJF:/var/lib/docker/overlay2/l/FJSNMHMQCRZYZ7DM6U7ZNTTDB3:/var/lib/docker/overlay2/l/VD3XKYF5UG747TAPJQARVNZQGS:/var/lib/docker/overlay2/l/FAM4UQVWPZY4VY7OWZYNQTYFKR:/var/lib/docker/overlay2/l/DJKXL7Y4XW736RZHCPHNPYBXST:/var/lib/docker/overlay2/l/EFQNH7TVKW2JE6WPQZ2GOI5IXL:/var/lib/docker/overlay2/l/7ZHB6OK5AAA3BEM5AHC55P52D5:/var/lib/docker/overlay2/l/Z3DKKP4J27SFFJWBJ3BRCFPOEC:/var/lib/docker/overlay2/l/FBREGCKO3ELPH4TDOKME5D7ZN5:/var/lib/docker/overlay2/l/DOVG72SJ7UVTP3NMAMBRVYFIK3:/var/lib/docker/overlay2/l/UT6LTWTRKNCSIVE3SN6KU5OBYD:/var/lib/docker/overlay2/l/IOYADTFJLVEG3E7P6YOT5TPEUM:/var/lib/docker/overlay2/l/3FFG3CZ65N42NXFHPBFS3ZG4DL:/var/lib/docker/overlay2/l/VROD4KBP7ADH5O5JKFHH7BSZJF:/var/lib/docker/overlay2/l/IRAV2XDBVUHXQ5YCOKUHOTI74A:/var/lib/docker/overlay2/l/JD6LPCYSY2ZYD5ZKTLF36S6H6U:/var/lib/docker/overlay2/l/7PM25KREJ3VRRD7AHYMUZO3GAP:/var/lib/docker/overlay2/l/TVAAGWQIIGNRRSW6CFVCTWNQEA,upperdir=/var/lib/docker/overlay2/cb73ac1031f0eb741fbcd882e7349a531a07f8dd5c8423252a4ea9708b8a9ee5/diff,workdir=/var/lib/docker/overlay2/cb73ac1031f0eb741fbcd882e7349a531a07f8dd5c8423252a4ea9708b8a9ee5/work 0 0