ext4: extended attribute value size limit is enforced by vfs

EXT4_XATTR_MAX_LARGE_EA_SIZE definition in ext4 is currently unused.
Besides, vfs enforces its own 64k limit which makes the 1MB limit in
ext4 redundant. Remove it.

Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>