linuxulator: Add linux_extattr_get_vp() for atomic getxattr

Move the atomic size-probe-and-read logic into a new
linux_extattr_get_vp() function in linux_xattr.c instead of
modifying the generic extattr_get_vp() in vfs_extattr.c.
This keeps Linux-specific getxattr semantics (ERANGE on
too-small buffer, EOPNOTSUPP to ENOATTR mapping)
self-contained within the linuxulator.

The function probes the attribute size and reads the data
under a single vnode lock, preventing a TOCTOU race between
the size probe and data read.

Signed-off-by: YAO, Xin <mr.yaoxin@outlook.com>
Reviewed by: kib
Pull Request: https://github.com/freebsd/freebsd-src/pull/2263